Owing to the electrification and the popularization of computer, there are usually a lot of electronic devices used in a house now. A problem of the power supply is then encountered. The consumed power of several electronic devices, such as speakers and modem, is not so much but each of these devices needs a specific voltage. Therefore, a rectifying device is necessary for this kind of electronic device. The volume of a conventional rectifying device is relatively big. However, the number of sockets in a house is commonly limited. Usually, two or three sockets are arranged together. That is, a rectifying device only provides one voltage but occupies two to three sockets in a time. When an electronic device is used, some sockets may be covered by the rectifying device and can not be used. When several electronic devices are used at the same time, the number of the sockets may not be enough. So, it is tried by the applicant to deal with the problem encountered in the prior art.